The present application relates to a process for decolorizing sugar juices using monodisperse ion exchangers and to the use of monodisperse ion exchangers for sugar juice decolorization. Preferably, monodisperse anion exchangers are employed for the inventive use.
For simplified production of high-grade sugars, improvement in yield or production of liquid sugar, substantial decolorization or desalting of the crude sugar solutions is customary. Thus, for example, relatively high color contents in the sugar syrup do not permit, without further work, the production of high-grade raffinates or water-clear liquid sugar syrups. However, provision of such sugar quality grades is now required by most consumers; e.g., as domestic sugar or in the drinks industry.
Sugar is produced from numerous plants. Of importance from the economic aspect are the production of sugar from sugar beet and cane sugar from sugar cane as well as from corn, wheat, basis rice, cassava potatoes or starch hydrolysates.
During sugar production, a crude sugar solution, which is termed thin juice or press juice, is obtained by extracting the beet cossettes with hot water or by pressing sugar cane. In addition to the sugar contents, it contains, depending on origin, varying non-sugar contents such as alkali metal ions and alkaline earth metal ions, chloride ions and sulphate ions, pyrrolidonecarboxylic acids and amino acids. During concentration of the press juices, other pigments such as caramel pigments and melanoidins are formed.
Colored constituents present in sugars are predominantly of anionic nature. There is a great number of different substances of which some are of high-molecular-weight nature. They can contain, for example, carboxyl groups, amino groups, phenol groups and other structural elements.
Sugar solutions can be decolorized, in the case of highly colored crude solutions (>1 000 ICUMSA) by precipitation methods based on carbonatation, sulphitation or phosphatation. Less-colored solutions (<1 000 ICUMSA) are decolorized either by physical processes, such as crystallization, or by adsorption processes using ion exchangers or activated carbon.
The color content of the solutions is determined by photometric measurement at 420 nm. The details are explained in the analytical methods. The unit for the color content is ICUMSA.
ICUMSA is equal to the product 1 000·Ecoe.
Ecoe is equal to the extinction coefficient.
To decolorize sugar solutions, bead-form adsorber resins based on crosslinked polystyrene/divinylbenzene or on polyacrylate are available. The adsorber resins are generally strongly basic anion exchangers of differing porosities. Depending on the application, either macroporous or gel types are preferably used. Depending on the pigment content, a single-, two- or three-stage process is employed. Combinations of the most varied ion exchangers based on acrylate and/or styrene/divinylbenzene on the one hand and macroporous and/or gel types on the other are conceivable.
Essentially two mechanisms are involved in the immobilization of colored sugar constituents on strongly basic anion exchangers: ionic interactions between anionic color components and the charges on the ion exchanger, and hydrophobic interactions between apolar parts of the color components and the styrene/divinylbenzene matrix—M. Bento, Int. Sugar JNL., 1998, vol. 100, No. 1191, page 111.
In U.S. Pat. No. 2,874,132, gel-type strongly basic anion exchangers containing quaternary ammonium groups based on styrene/divinylbenzene having divinylbenzene contents of 0.5 to 2% by weight are used for sugar juice decolorization. The anion exchangers are used in particular in mixed beds together with weakly acidic cation exchangers.
In U.S. Pat. No. 4,193,817, macroporous strongly basic anion exchangers containing quaternary ammonium groups in the chloride form based on styrene/divinylbenzene are used for sugar juice decolorization of cane sugars. The ion exchangers are packed into columns. At least two columns are connected sequentially in series.
An information publication from Rohm & Haas, amber-hi-lites, No. 108, November 1968, page 239, describes the use of strongly basic gel-type and macroporous anion exchangers for decolorizing cane and beet sugar solutions.
Macroporous anion exchangers and acrylic resins have a higher absorption capacity for pigment components and show a higher physical stability than gel-type anion exchangers in sugar juice decolorizations.
The efficiency of the bead-type adsorber resins is determined, inter alia, by the porosity, the internal surface area, the particle size and the degree of functionalization. Fine particles have a greater external surface area and as a result a better adsorption capacity. However, narrow limits are set owing to the high viscosity of the highly concentrated sugar syrups and the maximum permissible pressure drop which is very rapidly established on filtering the sugar solution through the adsorber resin bed. In contrast, coarse beads cause only a low pressure drop, but are distinguished by a lower adsorption capacity for the sugar colors.
The ion exchangers and adsorbers used according to the prior art are bead polymers having a broad bead size distribution (heterodisperse ion exchangers). The bead diameters of these adsorber resins are in the range from approximately 0.3 to 1.2 mm. The bead polymers underlying them can be prepared by known methods of suspension polymerization, see Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., Vol. A 21, 363-373, VCH Verlagsgesellschaft mbh, Weinheim 1992.
Owing to the presence of ion exchangers of different size, the beads exhibit different adsorption capacities for the pigments. This leads to a broad adsorption front and separation front.
An object of the present invention is therefore the search for suitable ion exchangers which avoid the disadvantages of the broad adsorption front and separation front and using which sugar juices of high quality and grade are obtained. The high quality and grade are exhibited in the lowest possible discoloration of the sugar juices.